install two products

Can you install a single use license on the same machine as a concurrent use license? I have a concurrent license with certain extensions but need other extensions. ESRI does not sell extensions for concurrent unless you are in a maintenance agreement and I need other extensions?

What do you mean it overwrote the previous version? When you install ArcGIS Desktop at the end of the install process the ArcGIS Administrator app appears and asks you to authorize ArcGIS and you specify a license manager or choose to authorize a Single Use license. Depending on which option you choose to do first will determine what you need to do next.